Process for the manufacture of boric acid

ABSTRACT

Boric acid is produced by treating a mixture of water and crude or calcined borocalcic ore with CO2, the partial pressure of which is at least 1 bar, at a temperature above 80* C. and then separating the liquid phase from the solid phase under a pressure and temperature at least equal to those under which treatment with CO2 was carried out. The pressure and temperature of the liquid phase are then decreased and the boric acid is crystallized from the liquid phase.

United States Patent Mathis [451 Apr. 4, 1972 [54] PRQCESS FOR THEMANUFACTURE 3,031,264 4/1962 Nies ..23/149 ACID 2,130,065 9/1938 Burkeet al .23/59 x 1,108,129 8/1914 Burger .23/149 [721 Invent: Mam,DmbasleeupMeufihe, 1,468,366 9/1923 Kelly ..23/59 France [73] Assignee:Solvay & Cie, Brussels, Belgium Examiner-Herbert Cartel b It d E 1.1. LFiled: p 1970 Attorney R0 e E Burns an mmanue obato [21] Appl.No.:33,291

[30] Foreign Application Priority Data Apr. 30, 1969 France ..6913966[52] U.S. Cl ..23/149, 23/59 [51] Int. Cl. ..C0lb /00 [58] Field ofSearch ..23/59, 149

[56] References Cited UNITED STATES PATENTS 725,786 4/1903 Smethurst..23/59 X [57] ABSTRACT Boric acid is produced by treating a mixture ofwater and crude or calcined borocalcic ore with C0 the partial pressureof which is at least 1 bar, at a temperature above C. and thenseparating the liquid phase from the solid phase under a pressure andtemperature at least equal to those under which treatmentwith CO wascarried out. The pressure and temperature of the liquid phase are thendecreased and the boric acid is crystallized from the liquid phase.

8 Claims, 4 Drawing Figures 70 80 2 3 g./kg.

Patented April 4, 1972 3,653,825

4 Sheets-Sheet 1 F gfi 90 2 as/ g.

50 pcozz 1.0

3 bars Patented April 4, 1972 3,653,825

4 Sheets-Sheet 2 Fig.2

Patented April 4, 1972 3,653,825

4 Shoots-Sheet :4

Fig.3-

80 B 0 g./kg.

Patented April 4, 1972 4 Sheets-Sheet 4 om I mm m mt N

PROCESS FOR THE MANUFACTURE OF BORIC ACID BACKGROUND OF THE INVENTIONThe present invention concerns improvements in the process ofmanufacturing boric acid by treating borocalcic ores with carbondioxide.

It is well known that boric acid can be recovered by crystallizationfrom aqueous solutions which are prepared by treating borocalcic oressuch as colemanite with CO in the presence of water. Most often,however, rather dilute aqueous solutions are obtained and aconcentration treatment must be carried out before separating boric acidby crystallization. One means to increase the concentration of B inaqueous solutions obtained by this process involves a prior calcinationof the ore as disclosed in U.S. Pat. No. 1,108,129 granted to A. Burger.One disadvantage of this process is that solutions are obtained whichhave a high content of dissolved CaO,i.e., from 5 to g./kg., when thecalcination is carried out at the recommended temperatures.

In order to obtain boric acid of sufficient purity, the solution mustnot contain too much dissolved CaO, otherwise other materials andparticularly calcium borates coprecipitate during the crystallization ofboric acid.

It has been proposed in U.S. Pat. No. 1,108,129 to neutralize thesolution of B 0 by adding a mineral acid before crystallizing the boricacid; however, if hydrochloric acid is used, the calcium is convertedinto CaCl which is soluble. Then, to eliminate CaCl the mother liquorsfrom the crystallization are simply discharged periodically. The calciummay also be eliminated by fixation on an adequate ion-exchange resin.This process has the advantage of not requiring the discharge of themother liquors of crystallization.

When sulfuric acid is used, calcium sulfate can be precipitated andseparated by decanting and filtering. But, in this case, crusting of theevaporators by CaSO .2H O takes place. Furthermore, part of the calciumremains dissolved and is coprecipitated afterwards with boric acid underthe usual conditions. The resultant boric acid then contains much morecalcium than the usual commercial product. In order to decrease thefinal content of dissolved CaO, the ore can be treated with silica inthe presence of CO as is recommended in British Pat. No. 863,541 grantedto U.S. Borax and Chemical Corp. Under these conditions, calciumsilicate is obtained as a by-product.

All of these processes indicate the necessity for decreasing the contentof dissolved CaO in boric acid before crystallization takes place andeach of these processes requires the use of a supplemental acid reactantand an increase in the number of treatments, which affects the economicsof the process.

SUMMARY OF THE INVENTION A new method has been discovered for treatingborocalcic ores whereby solutions which are richer in dissolved B 0 andwhich moreover contain less CaO are obtained.

According to the invention boric acid is manufactured by crystallizationfrom aqueous solutions obtained by treating a borocalcic ore with carbondioxide in the presence of water by a method which comprises:

agitating an aqueous dispersion of finely ground ore, crude orpreviously calcinated in at least one autoclave under a partial pressureof CO of at least 1 bar at a temperature superior to 80 C.,

separating the resultant solid phase from said liquid phase underconditions of temperature and pressure at least equal to those of thetreatment of ore with CO then decreasing the pressure, discharging theliquid phase and crystallizing the boric acid from the liquid phase.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram which shows thevariation of concentration of B 0 in solution obtained after varyingperiods of reaction of CO with an excess of crude colemanite in aqueoussuspension at temperatures of 90 C., 120 C. and 140 C., respectively.

FIG. 2 is a diagram showing the partial pressure of CO, at varyingtemperatures of reaction with at least stoichiometric amounts of crudecolemanite in an aqueous suspension wherein final solutions are providedwhich contain at least 60 g. of B O Jkg.

FIG. 3 is a diagram which shows for final solutions containing varyingamounts of B 0 the minimum temperatures at which reaction of colemanitestoichiometric amounts of calcined with CO under a pressure of at least2 bars must be carried out to prevent precipitation of calcium borate.

FIG. 4 is a schematic representation of a two-stage process of thepresent invention.

DESCRIPTION OF THE INVENTION TABLE I Temperatures C. 70 90 120 140 I70Concentrations of 13,0, in the final solution g./kg. 55 65 74 91 Theconcentrations of B 0 recorded in Table l were measured after thecolemanite had been treated with CO for at least 2 hours. After thisperiod of treatment, the concentration of the solution was found toremain substantially constant. This is shown in the diagram of FIG. 1wherein the period of treatment in hours, as the abscissa has beenplotted against the concentration of B 0 in g./kg. as the ordinate, forreactions at temperatures C., C. and C. under a partial pressure of COof 7.3 bars.

Identical tests were carried out with an excess of calcined colemanitein aqueous suspension; the results are recorded in Table 2 as a functionof the temperature under a constant pressure of CO equal to 7.3 bars.

In principle, a partial pressure of CO below 1 bar may be used if thetemperature is sufficiently elevated. Nevertheless, from a practicalpoint of view it is preferred to maintain a partial pressure of CO equalto at least 1 bar so that higher water vapor pressures are avoided. Highwater pressure is generally undesirable due to the resultant equipmentproblems, i.e. the equipment must be built to withstand considerablyhigher pressures if CO partial pressures substantially less than 1 barare used.

Every other condition remaining identical, when the temperature oftreatment is raised and each of the final solutions is separated underconditions of temperature and pressure at least equal to those of thetreatment, a decrease in the amount of CaO dissolved in the finalsolution is found..

Thus, reaction with CO has been carried out respectively on crudecolemanite under p 6 bars and calcined colemanite underp 7.3 bars. Theamount of ore has been calculated in order to achieve a concentration of65 g. of B 0 per kg. of final solution. The concentrations at differenttemperatures of treatment are recorded in Table 3.

TABLE 3 Temperatures of Treatment C. 50 90 120 I40 160 Crude colemanite:

Concentration of CaO gJkg. 2.5

Calcinatcd colemanite:

Concentration of CaO gJk 6.3 3.0 2.0 1.6

TABLE 4 Excess of crude Oolemanite Composition of the recovered clearliquor Decantation after de- Decantation under the creasing the pressureto Absolute pressure of the reactor 1 atmosphere total Temperapressure.B 103 (32.0 B 203 C20 ture. 0. bars g./kg. g./kg. gJkg. g./kg.

All the ores of the borocalcite type may be used according to theinvention such as, for example, colemanite, ulexite, pandermite,boronatrocalcite and boracite.

Advantageously, it is preferred that the recovered final solutioncontain at least 60 g. of B 0 per kg. in order that a suffrcient, andindustrially practical yield of H 80 is obtained during the subsequentcrystallization. Industrially, crystallization is carried out at about25 C., at which temperature the solubility of H 80 is about 30-40 g. B O/kg. If the solution contains at least 60 g. B o lkg, the yield fromcrystallization is at least 33 percent. This concentration, i.e., 60gJkg. is about the limit above which the mother liquors from thecrystallization of H 80 can be recycled to the treatment stage withoutthe ratio of the amount of treated solution to the amount of recoveredboric acid becoming too high.

When crude colemanite is used as the starting material, final solutionscontaining at least 60 g. of B 0 per kg. are obtained when thetemperature is comprised between 80 and 200 C. and when the partialpressure of CO is such that a point representing the operativeconditions on the diagram of FIG. 2 is always above the curve. On thediagram of FIG. 2, the temperature in C. is plotted on the abscissa andthe pressure in bars on the ordinate for reaction of at least astoichiometric amount of crude colemanite in aqueous suspension with C0to provide final solutions which contain at least 60 g of 8.

When calcined colemanite is used as the starting material, the sameresult is obtained when the temperature is maintained between 80 and 200C. and when the partial pressure of CO is at least 2 bars.

In both cases, it is preferred to operate at a temperature under 200 C.in order to limit the water vapor pressure.

Starting from crude colemanite, boric acid can be easily crystallizedfrom the recovered solution without neutralizing the dissolved CaO whichit contains, when the conditions of the diagram of FIG. 2 are respected.

Starting from calcined colemanite, the same result is achieved when l.the amount of ore is calculated so that no ore remains after thetreatment, and

2. if the point on the diagram of FIG. 3 corresponding to thetemperature of treatment and the content of 8,0, of the solution at theend of the treatment is above the curve; on diagram 3, the temperaturein C. is the ordinate and the final content of H 0 in g./kg. is theabscissa. When these conditions are not observed, there is acoprecipitation of calcium borates.

It has been found that above 120 C., the precipitation of calciumborates from the recovered solutions in unlikely since theirsolubilities are high. But, at about C. the rate of precipitation ofcalcium borates is high. Below 60 C., these borates precipitate veryslowly.

For this reason, to separate the boric acid from aqueous solutionsobtained according to the invention, it is preferred to cool thesolution rapidly to a temperature below 60 C.. Generally crystallizationis carried out at about 25 C. for economic reasons, however, this can bedone for instance, by flashing under vacuum.

The treatment can be carried out in one or more autoclaves continuouslyas well as discontinuously.

It has been found that the granulometry of the ore does not affect theconcentration of B 0 and CaO in the final solution. Thus, thetemperature of crystallization is not critical and any convenienttemperature below 60 C. may be chosen. At constant and temperature,using ore which has been granulated to dimensions below 0.125 mm.,between 0.160 and 0.250 mm. and below 2 cm., in each case, the sameconcentrations of B 0 and CaO are obtained. However, the granulometryinfluences the rate of reaction; finely ground ore reacts faster.Satisfactory rates of reaction are obtained with ore which has beenground to a particular size below about 1 mm.

While the tests described above have been carried out with an excess ofcolemanite, colemanite in any stoichiometric quantity or less whichyields a 8,0; solution of the desired concentration may be used. Theexcess of colemanite, whether crude or calcined, has no influence on thedesired maximum concentration of B 0 in solution, which is a function ofthe temperature and CO pressure in the reactor.

The amount of water employed per kg. of ore is determined from thedesired concentration of B 0 in the solution-and from the content of B 0in the ore. Usually, the colemanite in present use contains 41.5 to 46.5percent by weight of 8,0, and the calcium borate in this ore has theformula 2CaO.3B 0 .511 0.

In order to take the water of crystallization (SI-I 0) into account:

1 g. B 0 in the ore s 1.43 g. (B 0 water of crystallization) in thesolution Thus, if a solution containing 60 g. B O /kg. is desired, theratio (w/w) which may be used in the reactor is the following Water B 0in the crude ore For a colemanite containing 46 percent (w/w) of 8,0;the finaltatie k... W

Water The following examples further illustrate the best mode currentlycontemplated for carrying out the present process but they must not beconstrued as limiting the invention in any manner:

EXAMPLE 1 Into a 3 l. autoclave of stainless steel provided with adouble envelope for heating with a thermostatic fluid and with animpeller, having 2 vanes inclined at 45, a breadth of 20 mm.

which rotates at 400 r.p.m. are introduced:

2,500 g. of water 439 g. of pulverized crude colemanite, the dimensionsof the grains of which are less than 1 mm.; theoretically, the solutionwould contain 71.1 g. of B 0 per kig. if all the B 0 were dissolved;

CO the pressure of which in the autoclave is controlled by a monometercoupled to an expansion valve.

The pressure of CO chosen is 7.3 atm. and the temperature is 140 C.

Several samplings of clear liquor are made under pressure during thetreatment by means of a pipe plunging into the autoclave. The followingconcentrations are determined after several periods of time:

When the reaction is completed the agitation in the reactor was stoppedin order to separate the solid and liquid phases by decantation. After 1hour, the solution is drawn off and rapidly cooled by a refrigerant toabout 90 C.

The recovered final solution is concentrated enough to enable theseparation of boric acid by crystallization normally. Moreover, theconcentration of CaO being low, any chance of precipitating calciumborates is avoided provided the crystallization is carried out withcertain precautions as mentioned above; i.e., H BO is crystallized byfurther rapid cooling to about 25 C. In this way 144.5 g. ofcrystallized H BO is obtained.

EXAMPLE 2 The process was carried out in two stages and will bedescribed with reference to FIG. 4.

In the mixer M of 25 l. were introduced:

8.34 kg./h. of colemanite (440 g. B O /kg., 236 g. CaO/kg.)

via the inlet 1 at a temperature of 20 C.

82.8 kg./h. of an aqueous solution comprising the mother liquor of thecrystallization of boric acid obtained in the drying-machine ES. Thissolution enters the mixer M by way of conduit 3 and it contains 42.2 g.B O /kg. and 2.62 g. CaO/kg.

32.7 kg./h. of an aqueous solution obtained in the filter F where theresidues of the ore are separated. This solution contains 55.1 g. B O/kg. and 0.8 g. CaO/kg. and enters the mixer M by way of conduit 4.

From the mixer M, the suspension of colemanite is sent to the reactorR1.

The four reactors R1, R2, R3, R4 each of 100 l. are maintained under aCO, pressure of 11 bars. CO is introduced to the reactor R3 at a rate of1.52 kg./h. via inlet 2. Means for agitation a1, 04, are provided inthese reactors. The temperature in each of the reactors is maintained at140 C.

After reactor R3, the liquid phase of the reaction medium contains 65 g.B O /kg. and the suspension consisting of this liquid phase and of theresidual solid phase of the ore is introduced by means of conduit 5 tothe decanting tank D at a rate of 124.8 kg./h. at 11 bars and 140 C. Thedecanting tank D is maintained under conditions substantially the sameas those of the reactors, except that no agitation means are providedwithin.

From decanter D, two parts are obtained:

1. By way of conduit 10, 107.4 kg./h. of a liquid phase containing 65g.B O lkg. and 2 g. CaO/kg. which is introduced at 140 C. to theevaporator EV.; after that, via conduit 1 I, 88.3 kg./h. of a solutionwith 79 g. of B O lkg. is introduced at 65 C. to the crystallizer C1,the temperature of which is 40 C. and volume 20 1. From the crystallizerC1, 88.3 kg./h. of a mixture of crystallized l'l BO solution isconducted via conduit 12 to the crystallizer C2 of 20 l. maintained atC.; this mixture contains some crystallized H and a first mother liquorcontaining 57.5 g. of B,O /kg.; this mixture is then sent to thecrystallizer C2 maintained at 25 C.; a second mixture escapes viaconduit 13 from crystallizer C2, at the same rate of 88.3 kg./h. and itcontains the total amount of crystallized H BO and the finalmotherliquor which contains 42.5 g. B O /kg. This mixture is washed andseparated at 25 C. in the drying-machine ES from which are obtained:

6.5 kg./h. of H BO containing 536 g. of B O /kg.,

82.8 kg./h. of a solution comprising the 'mother liquor and the washingwater which is returned via conduit 3 to the mixer M.

2. By way of conduit 6, the residual suspension of ore enters thereactor R4 maintained at 140 C. under a pressure of 11 bars in which thetreatment of the ore is carried out. The reactor R4 receives 17.4 kg./h.of this suspension, in which the liquid phase also contains 65 g. ofBgOa/kgand 2 g. of CaO/kg. From the reactor R4 is obtained, via conduit8, 41.1 kg./h. of a suspension in which the liquid phase contains 55.1g. of B O /kg. and 0.7 g. of CaO/kg. The suspension is sent to theexchanger E and then to the filter F from which is obtained 8.3 kg./h.of the residue of the ore (via conduit 9) which essentially comprisesCaCO and 32.7 kg./h. of a solution containing 55.l g. of E o /kg.

which is returned in the mixer M.

Thus the amount of B 0 finally obtained in the form of boric acidrepresents 95 percent of the weight of B 0 introduced in the form ofcolemanite, without any risk of the precipitation of calcium borate.

While the present process has been illustrated by the foregoing specificexample, it is evident that many modifications can be made thereinwithout departing from the scope of the invention. For example,borocalcic ores in general can be treated by the present processincluding ore having a considerable lower B 0 content than that of thespecific colemanite referred to herein, and the pressure of CO used inthe process can be substantially lower or higher than that illustrated.

What I claim as new and desire to secure by Letters Patent is:

1. Process for the manufacture of boric acid by crystallization fromaqueous solutions obtained by treating borocalcic ore with C0 in thepresence of water which comprises agitating an aqueous dispersion offinely ground crude borocalcic ore under a partial pressure of CO of atleast 1 bar at a temperature of about -200 C., and in which the pressureof CO is such that the point corresponding to the pressure of CO at thetemperature of treatment is above the curve drawn on the diagram of FIG.2,

separating the solid phase from the liquid phase under a, pressure and atemperature at least equal to the pressure and temperature under whichtreatment with CO is carried out,

decreasing the pressure and temperature of said liquid phase, and

crystallizing boric acid from said liquid phase.

2. Process according to claim 1 in which an aqueous dispersion of crudecolemanite is treated with CO 3. A process according to claim 1 in whichthe proportion of water and ore in said aqueous dispersion is that whichyields a concentration of at least 60 parts of B per kg. of said liquidphase.

4. Process according to claim 1 in which the borocalcic ore has aparticle size of less than 1 mm.

5. Process according to claim 1 in which after the solid phase isseparated from the liquid phase, the liquid phase is cooled rapidly to atemperature below 60 C. by flashing under vacuum.

6. Process for the manufacture of boric acid by crystallization from anaqueous solution obtained by treating borocalcic ore with CO in thepresence of water which comprises agitating an aqueous disperson offinely ground calcined borocalcic ore, the exact amount of ore necessarybeing employed so that no ore is left after treatment with COmaintaining the pressure of CO above 2 bars.

maintaining the agitated aqueous dispersion at such a temperature thatthe point corresponding to this temperature and the final concentrationof B O in the solution is above the curve of the diagram of FIG. 3,

separating the solid phase from the liquid phase under a pressure and atemperature at least equal to the pressure and temperature under whichtreatment with C0 is carried out,

decreasing the pressure and temperature of said liquid phase, and

crystallizing boric acid from said liquid phase.

7. Process according to claim 6 in which the proportion of water and orein said aqueous dispersion is that which yields a concentration of atleast 60 parts of B 0 per kg. of said liquid phase.

8. Process according to claim 6 in which said borocalcic ore is calcinedcolemanite.

2. Process according to claim 1 in which an aqueous dispersion of crudecolemanite is treated with CO2.
 3. A process according to claim 1 inwhich the proportion of water and ore in said aqueous dispersion is thatwhich yields a concentration of at least 60 parts of B2O3 per kg. ofsaid liquid phase.
 4. Process according to claim 1 in which theborocalcic ore has a particle size of less than 1 mm.
 5. Processaccording to claim 1 in which after the solid phase is separated fromthe liquid phase, the liquid phase is cooled rapidly to a temperaturebelow 60* C. by flashing under vacuum.
 6. Process for the manufacture ofboric acid by crystallization from an aqueous solution obtained bytreating borocalcic ore with CO2 in the presence of water whichcomprises agitating an aqueous disperson of finely ground calcinedborocalcic ore, the exact amount of ore necessary being employed so thatno ore is left after treatment with CO2, maintaining the pressure of CO2above 2 bars. maintaining the agitated aqueous dispersion at such atemperature that the point corresponding to this temperature and thefinal concentration of B2O3 in the solution is above the curve of thediagram of FIG. 3, separating the solid phase from the liquid phaseunder a pressure and a temperature at least equal to the pressure andtemperature under which treatment with CO2 is carried out, decreasingthe pressure and temperature of said liquid phase, and crystallizingboric acid from said liquid phase.
 7. Process according to claim 6 inwhich the proportion of water and ore in said aqueous dispersion is thatwhich yields a concentration of at least 60 parts of B2O3 per kg. ofsaid liquid phase.
 8. Process according to claim 6 in which saidborocalcic ore is calcined colemanite.